04 RX330, 2WD, 110K

I cannot figure out what more to do. I replaced the cat with a lexus cat in Texas a few months ago. Since then I've changed a battery, replaced the oil, changed blades and rotated the tires. Finally I shipped the car to WA. The first month was OK, then I drove it to snoqualimie for a day where it is cold. On the way back the CEL, VSC, Trac OFF lights came on. I scanned it with P0420 but decided to clear it anyway and see if it comes back. A week or so later, all three were back on and it is scanning the same code again.

The last time I had this code, the consensus was change the cat, which I did and it ran for several months w/o the CEL coming on, so I figured that was it. I suppose it is possible the cat is bad again, but seems odd. I don't see anything obvious loose or disconnected.

(I realize most of the service has nothing to do with this, but its for the sake of completeness).

 

So I did one more thing and fired up torque to monitor the sensors. What I find as I accelerate and decelerate is that the O2 sensor before the cat voltage goes up and down as expected. The downstream sensor seems to be right at a constant voltage of 0.6 to 0.7V -- I thought that is the correct behavior.

 

unless you want to take it to the dealer for a diagnosis by a professional, your only option is trial and error.

next parts to be changed in my opinion would be the oxygen sensors, i would replace both and go from there.

oxygen sensors believe it or not really only have a lifespan of about 100,000 km (60,000 miles). just because there isnt a direct link to them doesnt mean they are not warn and need replacement.

 

Pull the code. I can help you from there. I need the code and to see if it is running lean or rich. Look at your short term and long term fuel trims and let me know what they read.

 

Also search my posts for the P420 post I made a while back. It shoudl have read P0420.

 

I looked myself but could only find info on your P0171 and P0174 error code problems.

https://www.clublexus.com/forums/rx-...074-codes.html

 

The code is still P0420 ....

 

Check your O2 sensors. My bet is you have at least one that is bad.

Usually the POST cat O2's are the one's that go bad or give the improper readings. However, could be the pre cat ones easily. You have 2 pre cat O2's. One is located near the fire wall just after the exhaust manifold, and one is located towards the bottom of the engine by following the exhaust manifold from the front of the engine down.

 

Do you know which O2 sensor is bank 2 sensor 1? The connectors are different. One is male and one is female.
Thanks!

 

Sensor 1 is ALWAYS PRE CAT CONVERTER. You have one that is in the front of the engine near the radiator on the exhaust manifold. The other is on the rear exhaust manifold near the fire wall. Only accessible through the bottom of the engine going up.

 

Thanks, let me give that a try next weekend. I seem to recall that by monitoring the voltage of them through the ODB2 that it was possible to determine which one(s) are good/bad. But I think last time I did it, I did not run the car long enough so the cat wasn't hot enough to work. Is that a workable method for evaluating which one is broken?

 

Yes you can use a reader to gather that data. Car has to be up to full temp and usually driving not just idlng. It is very important to know the proper voltage ranges though when evaluating this. A reading does not mean much. The proper ranges are what matters.